Demand for electric vehicles with hybrid drive has soared worldwide due mainly to a recent sharp increase in fuel prices. As oil reserves continue to dwindle and oil prices rise, and with no viable alternative fuel technologies becoming apparent, the demand for electric vehicles with hybrid drive will only increase. According to a recent survey, in 2008 alone, 36.0% motorists worldwide want to buy a car with hybrid drive while 45.8% are interested in buying full-electric cars. Electric cars are powered entirely with electrical energy from tens of thousands of battery cells. These battery cells are grouped and assembled as a set of battery packs. Individual cells in a pack, which are exposed to, and must operate in a harsh environment, have different operating characteristics due to difference in their manufacturing tolerances, uneven temperature conditions across the pack, or non-uniform ageing patterns. These varied settings, in turn, have crucial effects on the charge/discharge of battery cells. In a series chain of battery cells, a weak battery cell with low capacity reaches its full charge state well before the rest of the battery cells in the chain, hence overcharging and overheating itself. On the other hand, when the weak cell cannot reach its full charge owing to a high self-discharge and/or a short-circuited cell, good battery cells may overcharge. In a series chain of battery cells, an open-circuited cell causes the others in the chain to become open-circuited as well. All of these phenomena eventually lead to a battery-cell failure, which is inevitable especially in large-scale battery packs.
The most commonly-used method for managing a large-scale battery system is module-based, where battery cells are grouped into smaller modules of battery cells, each of which is monitored, controlled, and balanced by the corresponding local controller while a group of modules are managed by a global controller. In such a modular battery-management system, individual electronic control units (ECUs) collect information-such as cell voltage and current, temperature, etc. on their serially-connected battery packs via an equalizer connected to each battery cell, and then process and report the collected information to the central ECU responsible for making the local ECUs work as required.
Individual battery cells can be charged and discharged separately via the switches around them. Separate discharge or bypassing specific cells, however, requires fine-grained management of battery cell arrangement and battery dynamics while considering their attributes (e.g., cell voltage balancing and capacity efficiency).
Battery-cell failures are inevitable, especially for large-scale batteries, and the failure rate for a multi-cell battery pack is much higher than that of each cell because of inter-cell interactions and dependencies. Unlike the battery packs used for portable electronic devices, the electronic vehicle environment imposes many challenging requirements on battery cells and their management.
There are two main challenges in developing a dynamic reconfiguration framework for large-scale battery-management systems. First, the framework should be able to reconfigure battery connectivity online, upon detection of a battery-cell failure. Healthy battery cells should also be kept in use, possibly in the form of two hierarchical layers of connectivity: battery cells in each pack (cell-level) and packs in the entire battery system (pack-level). Second, unlike battery-powered portable devices, a large-scale battery-management system, especially for electric vehicles, requires multiple output terminals of the power source (from the battery packs), supplying different voltages for different applications and/or devices. Physical separation of battery packs is, however, rarely an option mainly for cost reasons.
This section provides background information related to the present disclosure which is not necessarily prior art.